1. Field of the Invention
The invention relates to a calibration device for calibrating the orientation parameters of digital optoelectronic sensors arranged in a carrier such as a aircraft or a satellite for remote reconnaisance such as photogrammetry.
2. Description of the Related Art
Digital sensors are already playing a significant role in remote sensing today due to their characteristic of recording data in digital form and then transmitting the data to a receiving station. Due to the developments in the field of semiconductor electronics, digital sensor systems will also be capable of advancing within a foreseeable time into the field of traditional photogrammetry and replacing the analog pick-up systems such as, for example, aerial cameras which are based on film.
For the evaluation of the sensor data especially in the case of photogrammetric tasks, considerable importance attaches to the knowledge about the attitude and the position, i.e., orientation parameters, of the pick-up system. It is only if these orientation parameters are known that geometric statements can be made about the observed objects. The accuracy of these so-called orientation parameters determines the quality of the measured geometric variables. In this context, the outer orientation parameters specify the attitude and position of the center of projection of the pick-up system and the inner orientation parameters describe the relationship between the center of projection and the image plane.
The orientation parameters for analog aerial photos are generally determined with the aid of register points, i.e., points on the ground having known locations, during image analysis. The coordinates of these register points are surveyed accurately. The determination of the orientation parameters has been supported in recent years by using GPS and navigation systems.
This traditional approach presents special problems in the determination of the orientation parameters for image data which have been picked up by line scanners. Due to the fact that a rigid image geometry is lacking, the orientation parameters must be redetermined each time an image line is picked up by a line scanner.
A prior art parameter determination method is disclosed, for example, in German reference DE 38 02 219 A1 wherein a series of overlapping individual pictures is made at regular time intervals by an optoelectronic planar pick-up camera allocated to one or more line scanners or a combination of such cameras. The positions of corresponding picture elements or picture zones in the overlapping area of adjacent single planar pictures are in each case selected for the mutual orientation of the pick-up positions of the planar camera. The orientation data of the optoelectronic planar camera is transmitted by permanent alignment or measurement of the relative alignment to the line scanner supported by a time correlation between both data streams. To carry out the method, planar detectors are preferably arranged in the same focal plane between the line scanners.
As the orientation parameters obtained by conventional methods of the photogrammetry always refer to the center of projection, some problems occur during the measuring of these variables. If the position is determined by GPS, the position of the GPS antenna is determined, as a rule. The three angles of rotation are also not measured with respect to the center of projection of the sensor. Moreover, gyroscopic systems only register changes in the angle of velocity, as a rule. To determine an absolute angle, reference angles are necessary at least at one point in time. All six parameters of the outer orientation parameters are thus subject to an offset. These variables are not known and must be determined by additional measurements such as, for example, by theodolites or by evaluating register points. This results in additional expenditure for measurement technology and in additional error sources. Furthermore, this method prevents complete on-board processing.
An arrangement for obtaining highly accurate data from the air is disclosed in German reference DE 43 14 742 A1 which comprises a sensor system having at least non-imaging sensors for determining anomalies of the conductivity and/or disturbances in the earth magnetic field in the area or, respectively, ground to be probed, and a flight guidance system which operates in conjunction with a differential position determining system, the sensor and flight guidance system being arranged in a helicopter. The differential position determining system includes a GPS reference station arranged at a fixed location with known coordinates in the vicinity of the area to be probed. The GPS reference station operates in conjunction with a DGPS transmitter for transmitting position correction data to a DGPS receiver in the helicopter.
A surveying instrument is disclosed in German reference DE 26 38 621. The surveying instrument includes a device for measureing angles such as a theodolite in which the values measured by this device such as the elevation angle or horizontal direction are dependent on leveling errors during the installation. Accordingly, a device for determining leveling errors of the equipment may be permanently connected to the equipment, wherein a computing unit which calculates from the measurement value and leveling error the value of the corrected measured variable, the leveling error being determined by the output signal of a Schottky barrier diode which is mounted rigidly on the equipment and to which a light ray reflected from a liquid surface is applied.